Conventionally rotary winches have a single drum to store the rope and to apply the required force to the rope. This type of winch is commonly used for the installation of mooring lines. A mooring line may be a steel spiral strand wire, a polyester line or any other line of suitable material. Another application of rotary winches is for the installation of umbilicals. In the text the term “line” is used for any kind of elongate member which can be used on a winch.
In current practise of offshore applications, lines are transported from shore to an installation vessel on so called storage drums. These are drums that in general have a diameter that is minimized for the type of line that is transported in order to achieve the most efficient and compact way for storage and transportation purposes. Since there is only limited tension in the lines using a small diameter drum for this purpose is no problem. Also the line can be spooled on the storage drum in multiple layers. Interaction between layers is small since the tension in the line is small.
After arrival on the installation vessel the line is spooled on a larger winch drum for installation. This larger winch is referred to as the mooring line deployment winch. During installation high tensions may occur in the line.
The mooring line deployment winch is normally used to lower and pick up lines to or from the bottom of the sea with the installation vessel. During installation of these lines it is preferred to load only a single layer on the drum. When more layers are used, locally very high tension in multiple layers can occur. The top layer may force itself in between lower layers, especially at the flanges, where the top layer transits to a lower layer. Also, because of the winding on the drum there will be points where the top layer has very small contact area with only one cable of the lower layer, thereby introducing very high tensions in both layers. This can cause damage to the line during unreeling.
Due to the increasing water depths in which these lines have to be installed, the situation arises that the lines which have to be deployed by the mooring line deployment winch become too long to fit in a single layer on the drum.
It is remarked that systems are known which are configured to guide spooling wires in multiple layers so that the different layers are positioned in the most suitable manner on top of each other. A well known example for such a system is the so called Lebus groove. Reference is made to patent U.S. Pat. No. 2,620,996. Although such systems have proven to work, in practise the local tensions occurring in the line may reach unacceptable levels, for instance when fibre ropes are being handled. Moreover, the length of the line to be used in conjunction with these systems is limited.